1. Field of the Invention
The present invention relates to the art of a control system for a vehicle having an engagement mechanism that transmit torque selectively between an engine and a driving wheel, and that governs a torque transmitting capacity.
2. Discussion of the Related Art
Hybrid vehicles typically use an electric motor or a motor-generator (as will be simply called a “motor” hereinafter) as a prime mover together with an engine. The hybrid vehicle of this kind has various advantages over conventionally known vehicles. For instance, the engine is allowed to be stopped temporarily when the hybrid vehicle comes to stop (that is, idle stop), and the motor is allowed to regenerate electric energy during decelerating the hybrid vehicle. In addition, the engine is also allowed to be driven at an optimal driving point to improve energy efficiency. A fuel economy of the hybrid vehicle thus structured is improved by using the motor mainly to drive the vehicle while stopping the engine to avoid consuming fuel. To this end, the engine has to be disconnected from a powertrain including the motor outputting driving force to the driving wheel so as to reduce a power loss resulting from rotating the engine concurrently. A hybrid vehicle comprised of a clutch for disconnecting the engine is described in JP-A-8-295140. In the hybrid vehicle taught by JP-A-8-295140, therefore, the engine is allowed to be stopped and rotated selectively in the case of driving the vehicle by the motor.
According to the teachings of JP-A-8-295140, the first gear element of the differential gear unit is coupled to the generator, the second gear element serves as the output element, and the third gear element is coupled to the breaking means. The third gear element is also coupled to the engine via the clutch. That is, in the hybrid vehicle taught by JP-A-8-295140, the differential gear unit is allowed to serve as a speed reducing device or a speed increasing device by halting the third gear element by the braking means. In this situation, the generator coupled to the first gear element can be used as a motor, and a torque thereof is transmitted to the output element. Consequently, the vehicle is driven by the powers of the generator and the motor. Specifically, the braking means is a one-way clutch adapted to be engaged to halt the third gear element when the third gear element is rotated by the torque in the reverse direction. Therefore, during driving the vehicle by the motor, the third gear element may be halted by the one-way clutch independent of the engine and the clutch may be disconnected from the third gear element. That is, the engine is allowed not only to be driven but also to be stopped during driving the vehicle by the motor. Meanwhile, during driving the vehicle by a torque of the engine coupled to the third gear element via the clutch, a speed of the engine may be controlled by controlling a speed of the generator coupled to the generator. That is, the differential gear unit is allowed to serve as a continuously variable transmission in this situation.
In the hybrid vehicle in which the clutch is thus disengaged during driving the vehicle by the motor, the clutch may be engaged to carry out a cranking of the vehicle and to use the output torque of the engine to drive the vehicle. Meanwhile, an input speed and an output speed of the clutch may differ from each other while the vehicle is running. In this situation, if the clutch is engaged abruptly, the driving force would be changed significantly thereby causing a shock. Such a shock resulting from an abrupt change in the driving force may be avoided by engaging the clutch while changing an engaging pressure gradually, in other words, by engaging the clutch while slipping. However, the clutch may have a fever if it is engaged while slipping, and as a result, durability of the clutch may be degraded.
In order to avoid the foregoing disadvantage, the control device taught by JP-A-2010-190254 is configured to stop a slippage of the clutch compulsory when a predicted heat quantity is greater than the preset criterion. Specifically, according to the teachings of JP-A-2010-190254, the heat quantity of the clutch is calculated based on a required torque transmitting capacity of the clutch and a difference between an input speed and an output speed of the clutch. The control device of JP-A-2010-190254 stops a slippage of clutch by increasing an engaging pressure when an integrated value of heat quantity exceeds a preset criterion value.
In addition, JP-A-2009-132196 discloses a control apparatus of a hybrid vehicle in which the motor is connected to the output shaft of the engine through the first clutch, and driving wheels are connected to the output shaft of the motor through the second clutch. In the hybrid vehicle taught by JP-A-2009-132196, a drive mode can be selected from: an engine-used slip drive mode in which the first clutch is engaged and the second clutch is slip engaged; a motor slip drive mode in which the first clutch is disengaged while slipping the second clutch; and a motor drive mode in which the first clutch is disengaged and the second clutch is engaged. In order to reduce thermal deterioration of the second clutch, if a temperature of the second clutch exceeds a predetermined value under the engine-used slip drive mode, the control apparatus taught by JP-A-2009-132196 switches the drive mode to the motor slip drive mode or the motor drive mode where a slip of the second clutch is smaller than that under the engine-used slip drive mode.
Meanwhile, a pushing force of an actuator for the clutch is controlled in accordance with a required torque transmitting capacity of the clutch. For this purpose, if a friction surface of the clutch is worn with age so that a friction coefficient of the friction surface is changed, control accuracy of the clutch will be deteriorated. In addition, if an elastic member is also used to control the torque transmitting capacity of the clutch, such attrition of the friction face of the clutch changes a required elastic force to achieve a required torque transmitting capacity of the clutch. Thus, the temporal deterioration of the clutch resulting from wear of the friction face will influence the engagement property of the clutch, and this makes the clutch difficult to be controlled.